The invention relates to a circuit arrangement in which a radio transmission frequency is modulated by means of a baseband signal.
In order to generate digital modulation (FSK, PSK, QAM, etc.) the so-called I, Q or quadrature principle is typically applied, in which the sin and cos wave shapes of the carrier wave are multiplied by the sin and cos wave shapes of the modulating signal, after which the results are summed.
Analog frequency modulation is typically created by controlling, by means of a voltage corresponding to the modulating signal, a voltage-controlled oscillator generating the carrier wave frequency. Another method of generating analog frequency modulation is based on the I, Q principle, as is the digital modulation. The quadrature I, Q signal shapes are then formed from an analog reception signal. The generation of I, Q signal shapes is known e.g. from the FI patent application 843 826.
In applications using both digital and analog transmission channels (for instance the future US digital mobile phone system) it is advantageous that the same equipment be able to generate both the digital modulation and the analog modulation to be used.